A pyridazinone compound of the formula (1), i.e. 4-chloro-5-[3-(4-benzylpiperazin-1-yl) carbonylmethoxy-4-methoxybenzylamino]-3(2H)-pyridazinone is disclosed in WO95/01343 laid open on Jan. 12, 1995 and JP-A-8-041033 laid open on Feb. 13, 1996, and is a compound useful as a pharmaceutical product having a bronchodilation function, an antiallergy function or an antiplatelet function. ##STR1##
The above patent publications disclose a hydrochloride of the pyridazinone compound (1) and a method for producing the same, and a 2-hydrochloride (see Reference Example 1) obtained by the above preparation method has an excellent pharmaceutical effect. However, the 2-hydrochloride is highly hygroscopic and is easily decomposed and is rapidly decomposed in a methanol solvent. Thus, the 2-hydrochloride is unstable, and is therefore not suitable as a starting material for a pharmaceutical product.